1. Field of the Invention
The present invention relates generally to peptides bearing selected antigenic epitopes and to nucleic acid compositions which may be employed in the preparation of such peptides, or to detect the presence of complementary nucleic acid sequences in biological samples. More particularly, the invention concerns peptides bearing antigenic epitopes corresponding to epitopes on the 60 kD polypeptide of the Ro/SS-A antigen (also referred to simply as the "Ro" antigen), and to DNA sequences encoding all or a portion of the 60 kD polypeptide. The invention further relates to processes incorporating the foregoing peptides and/or nucleic acid sequences, such as in the immunocharacterization of various autoimmune diseases or in the preparation of recombinant Ro antigen.
2. Description of the Related Art
Patients with rheumatic diseases can make autoantibodies to a variety of biological compounds derived from their own cells, including autoantibodies to products secreted by cells (e.g., rheumatoid factor to immuno-globulins), constituents of the plasma membrane (e.g., phospholipids, insulin receptors) as well as an array of intracellular components. Interestingly, of the greater than 10,000 macromolecules found inside a cell, only about 30 are targets of autoantibody production (1). However, autoimmune antibodies having immunospecificity for one or more of these targets are found in a wide array of rheumatic diseases, including systemic lupus erythematosus (SLE), mixed connective tissue disease (MCTD), primary sicca syndrome, polymyositis, dermatomyositis, progressive systemic sclerosis (PSS), rheumatoid arthritis (RA), idiopathic thrombocytopenic purpura (ITP), primary biliary cirrhosis (PBC), chronic active hepatitis (CAH), and a variety of others.
Accordingly, the presence of autoantibodies in the serum of a patient is generally indicative of one or another of the various foregoing conditions. For example, autoantibodies to double-stranded (ds) DNA occur in 50-70% of SLE patients and are highly specific for this disease (2). This autoantibody is also occasionally seen in clinical settings where SLE overlaps with other rheumatic diseases (e.g., mixed connective tissue disease). Moreover, circulating ds-DNA antibody levels fluctuate with systemic disease activity, particularly renal involvement (3-4), and this autoantibody specificity has been implicated in the more aggressive forms of lupus nephritis (5-6).
The presence of one or more of a variety of other autoantibodies have been used as indicators of rheumatic disease, including antibodies having specificity for chromatin structural proteins such as histones or nucleosomal structures or antibodies to ribonucleoprotein particles (RNP) such as nRNP, U1 snRNP and Sm antigens. For example, studies employing solid phase immunoassays have shown that anti-histone antibodies can be found in about 50% of the sera of unselected SLE patients (7-9), and in approximately 80% of patients with active disease (9).
The appearance of antibodies to Ro/SS-A and La/SS-B RNPs were probably first detected in 1958 in the search of patients with Sjogren's Syndrome, employing extracts of salivary tissue as antigens (10). Later studies demonstrated two major specificities in salivary tissue designated SjD and Sjt (11), probably corresponding to Ro/SS-A and La/SS-B, respectively. Reichlin and Harley have offered several recent comprehensive reviews of the clinical correlations of the Ro/SS-A and La/SS-B antigen-antibody systems (12-13).
Most interest has centered on the Ro/SS-A system since an autoantibody response to this antigen is much more common than one to La/SS-B. In addition, an autoantibody response to La/SS-B is almost invariably associated with anti-Ro/SS-A antibody production. Anti-Ro/SS-A autoantibodies occur in the highest prevalence in Sjogren's syndrome (SS) patients. Moreover, some investigators have suggested that the use of a sufficiently sensitive solid phase assay employing purified Ro/SS-A antigen, virtually all SS patients produce this autoantibody (14).
Unfortunately, although purified Ro/SS-A antigen can be employed to immunodiagnose various autoimmune disorders and particularly Sjogren's syndrome, there are significant problems associated with its use. Of principal importance is the fact that although Ro/SS-A RNP particles can be isolated to some degree of purity (15), to do so is economically impractical. This is due principally to the high cost and time of isolating Ro/SS-A antigen proteins from natural sources. Moreover, in that the Ro/SS-A antigen is an RNP particle, it generally has stricter requirements for storage, as well as a more limited ability to prepare for commercial distribution. While a recombinant version of one or more protein subcomponents would prove to be useful in this regard, their development has not been deported. Clearly there is a need for such a recombinant version, including a need for DNA segments which can be employed in the preparation of peptides containing Ro antigenic sequences, which would provide a means for producing improved antigenic materials which may be recognized by antisera having specificity for autoimmune antigens such as Ro/SS-A.